Improvement of fuel economy in modern vehicles continues to be an important aspect of engine and transmission design. One known method of improving fuel economy is to implement a fuel cut mode when the vehicle is coasting, with the throttle closed, while traveling at relatively high speeds. The supply of fuel is cut off from the engine and the vehicle allowed to coast until the engine speed drops below a certain threshold or a request to accelerate the vehicle is once again made (the throttle is opened). If the vehicle is decelerating and has an automatic transmission, gear shifting, typically from fifth for fourth gears may occur while the vehicle is in a fuel cut mode.
As shown in FIG. 4, an automatic transmission includes a lock-up clutch 14, associated with a torque converter, between the engine crankshaft and transmission input shaft and a series of shift clutches 12 associated with numbered (1st-5th) transmission speed trains. The lock-up clutch 14 locks the engine crankshaft to the transmission input shaft as the vehicle speed reaches approximately 72-80 KPH. The transmission proceeds from lower gears (1, 2) to higher gears (4, 5) as the vehicle speed is increased and back down to the lower gears as the vehicle speed decreases. Each time the transmission advances upward or downward from gear to gear, an appropriate shift clutch is disengaged, then re-engaged, allowing a smooth shift.
During a shift from fifth gear to fourth gear, made in combination with application of the vehicle's brakes, overslip of the lock-up and shift clutches occurs and as a result the engine speed drops more than if such overslip did not occur (no brake application). FIG. 1 a shows the operating parameters of the vehicle during a shift from fifth to fourth gear with little or no deceleration caused by braking. Note, the fuel cut mode (FC) is able to remain on. FIGS. 1b and 1c illustrate the same operating parameters with moderate or high deceleration (DTV), respectively, caused by braking. As a result of dropping engine speed (NE), the fuel cut mode (FC) cannot be maintained for a desired length of time.
To satisfy Fuel Economy tests designed by the EPA, the fuel cut mode must be maintained during a shift from fifth to fourth gear when the vehicle is decelerating moderately. However, the fuel cut mode does not need to be maintained during high deceleration.
Referring to FIGS. 2a-2c, to remedy the inability to maintain the fuel cut mode, the clutch pressure of the lock-up clutch (LC) is increased by a consistent amount during shifting. The vehicle deceleration rates (DTV) represented in FIGS. 2a-2c corresponds to the deceleration rates of FIGS. 1a-1c, respectively. Additionally, the clutch pressure of the individual shift clutch(es) (e.g. 5th) is also increased by a consistent amount (typically not equal to the increased amount of the lock-up clutch) during shifting. Such pressure increases prevent significant engine speed drop (NE) and allow the fuel cut mode (FC) to be maintained during moderate deceleration (represented by FIG. 2b) and high deceleration (represented by FIG. 2c) wherein the fuel cut mode could not be maintained in instances of similar deceleration (FIGS. 1b and 1c). This is illustrated by comparing the engine speed line to the FC threshold horizontal line in FIGS. 2a-2c. However, the increase in lock-up clutch and shift clutch pressures detrimentally increases “shift shock”, a phenomenon caused by poor timing of sequential clutch disengagement of a shift clutch. Shift shock causes discomfort to the vehicle operator and is undesirable.
What is desired is a method of engine and transmission operation that allows the engine to remain in a fuel cut mode for an extended time, even during a downshift, while minimizing the phenomenon of shift shock. The viability of the method must be demonstrable during Fuel Economy testing designed by the Environmental Protection Agency (EPA), that includes a fifth to fourth downshift step while braking the vehicle.